Forming of metal articles, e.g., waveguides, using electroforming techniques, is known. Thus, in U.S. Pat. No. 2,826,524 to Molloy there is described an electroforming process for forming wave-transmitting elements, such as a hollow waveguide, a feed horn, etc., wherein initially an internal mandrel whose outside surface corresponds to the inside configuration of the waveguide is formed; a metal coating, which is formed by electrodeposition, is formed on the mandrel, reinforcing material is then provided on the metal coating, and the mandrel is then removed (e.g., by mechanical removal, or melting, or dissolution).
Problems arise, however, particularly when known electroforming techniques are utilized for forming metal articles on such mandrels having inner wall portions (e.g., slots, grooves, holes, etc.). In particular, there is a problem with providing sufficient material and uniformity on the inner wall portions to provide a final electroformed product with adequate strength and thickness, e.g., at the position corresponding to such inner wall portions. Thus, in the conventional electroforming method a heavy (i.e., thick) electrodeposit is caused to build up on the outside corners of the inner wall portions, that is, the high current density areas, while the electrodeposit remains thin on the inside walls.
British Pat. No. 685,247 describes a method of forming a metallic structure by electrodeposition on a mandrel, the mandrel having a sharp re-entrant angle, and describes the problem of depositing metal electrolytically in the sharp re-entrant angle since, in practice, electroforming on a structure including such angles results in thin, weak deposits in the angle and the formation of a deep crevice penetrating to the edge at which the walls forming the angle meet. This British patent attempts to overcome this problem by first electrodepositing a thin metallic deposit on the mandrel, including the sharp re-entrant angle, then packing the re-entrant corners with suitable metallic powder, and, after cleaning or preparation of the surfaces as may be necessary, then returning the mandrel to the plating bath to continue electrodeposition until the desired wall thickness has been built up. This British patent further discloses that the metallic powder with which the corners are packed may be precipitated silver which may be tamped into the corners.
As can be appreciated, even when the metallic powders are tamped into the corners, problems can arise with respect to adhesion of the layers and powder. Moreover, the final electroformed article would not be substantially uniform, among other reasons because the corners of the re-entrant angles would contain the metal powder. Furthermore, it is extremely difficult to utilize a method such as described in the British patent wherein metallic articles are formed on mandrels with inner wall portions having large depth-to-width ratios.
U.S. Pat. No. 2,898,273 to La Forge, et al. discloses an electrodeposition method for forming disc-loaded waveguides, wherein a plating core, i.e., mandrel, is formed by machining internal grooves into an aluminum cylinder, and then copper or silver is electroformed onto the plating core, and then the plating core is dissolved out. This patent discloses the problem of more rapid metal deposition at the outer than at the inner portions of the core grooves and the tendency for the plating metal to seal over the outer portions of the grooves before the grooves are completely filled with metal, which leaves cavities filled with entrapped plating solution; and describes an attempted solution to such problem of providing passages for escape of such entrapped plating solution by positioning threads of "Orlon" or the like transversely within the grooves of the plating core prior to or during the electrodeposition, and then burning the threads, to provide such passages, after the electroplating.
Thus, La Forge provides an overly thick coating at the outer groove portions as compared with the coating at the inner groove portions. Moreover, such procedure as described by La Forge would not solve the problem of providing sufficient coating thickness at the inner groove portions for grooves having a high depth-to-width ratio.
This problem of providing sufficient material and uniformity on the inner wall portions of the mandrel to provide a final article with adequate strength and thickness becomes more acute as the depth-to-width ratio of the inner wall portions become greater, especially as they become greater than 2:1. An example of an article to be formed utilizing mandrels with such large depth-to-width ratios are corrugated waveguide horns for extremely high frequency (EHF) application, e.g., millimeter corrugated horns. Such corrugated horns are very small and have very tight tolerances. These horns, and uses therefor, are known; see, for example, U.S. Pat. No. 4,295,142, issued Oct. 13, 1981, naming Thiere, et al. as the inventors. These horns have corrugations which, as an example, can be about 250 mils long and 90-300 mils wide. Such horns would be made utilizing mandrels having inner wall portions (e.g., narrow slots or grooves), with a depth of about 250 mils and a width as low as 90 mils.